Effects of biological activity by abyssal benthic macroinvertebrates on a sedimentary structure in the Venezuela Basin

Abstract

Macrobenthic standing stock estimates from the Venezuela Basin were in agreement with those calculated from other ocean basins. Biomass and density values were significantly higher at a site characterized by hemipelagic sedimentation compared to two sites characterized by carbonate and turbidite sedimentation. The higher standing stock is probably related to the higher input of organic matter to that site. Vertical distribution of fauna suggests that sediment was well mixed to 6–8 cm depth at the carbonate and turbidite sites and to 10–12 cm depth at the hemipelagic site. Infrequent mixing probably extends to 30 cm at the turbidite site, 18 cm at the hemipelagic site and 10 cm at the carbonate site. Mixing rates were predicted to be the highest at the hemipelagic site where benthic standing stocks were highest. These mixing depths and rates were in general agreement with those determined by radiochemical methods and by visual and X-ray photographic observations. Gradients of decreased porosity and increased density were probably the result of biological activity (burrowing and tube dwelling) as opposed to effects of overburden pressure. Sediment shear strength was controlled by biological activity coupled with biologically mediated chemical bonding at the redox discontinuity layer. Destruction of sand-sized foraminiferan tests by faunal ingestion of sediments contributes to a reduction in mean grain size at the sediment surface.